We have demonstrated that verapamil reduced left ventricular outflow tract obstruction and improved exercise capacity and subjective symptomatology in hospital in such patients. To assess the chronic effects ov verapamil in hypertrophic cardiomyopathy, 227 patients whose lifestyle was unacceptable despite propranolol therapy were begun on oral verapamil in hospital. Fifty-nine percent of patients discharged on the drug have remained on verapamil for periods as long as 53 months. The major reason the drug was stopped in patients discharged on verapamil was because symptoms were unrelieved or recurred. Less than 2% of patients discharged on verapamil had the drug discontinued because of non-physiologic drug side effects. Nine patients have died while on chronic drug treatment. Eight of these deaths were related to cardiovascular events but whether they occurred because of or despite verapamil therapy is uncertain. Adverse hemodynamic effects experienced by the patients include 27 episodes of pulmonary congestion, 12 of hypotension, 5 cases of sinus arrest, approximately 13% incidence of development of junctional rhythm or Wenkebach second degree heart block. In almost all cases of hypotension, junctional rhythm, second degree heart block, and non-cardiovascular problems, the drug was continued at reduced dosage, although therapeutic efficacy was sometimes compromised because of an inability to use higher doses. We have been able to identify those subgroups of patients who appear to be at increased risk for developing complications secondary to verapamil administration, and who should not receive the medication. In addition, it appears that its primary mechanism of therapeutic action may be due to its inability to improve early filling of the ventricle.